This invention relates to vinyl ester oligomers which carry a terminal sulfonate group and their hydroxyl containing derivatives. In another aspect it relates to a process for making such polymers. In still another aspect it relates to a method of using bisulfite chain transfer agents in the polymerization of vinyl esters in order to form low molecular weight, sulfonate-terminated oligomers that can be used as is or converted to oligomers containing vinyl alcohol groups and having a terminal sulfonate group.
It has become increasingly difficult to find suitable surfactants and surface-active agents to meet the vast variety of needs found in industry today. These materials are required in products such as soaps, detergents, emulsifiers, dispersion and suspension stabilizers, paper coatings, inks, pigment dispersants and grinding aids, papermaking additives, flocculents, and the like. Each of these uses has special requirements to satisfy its particular application. It would be highly desirable to be able to meet these needs by preparing low molecular weight polymers or oligomers from available monomers that polymerize readily in aqueous systems with free radical initiation. In such systems, however, molecular weight control is a problem because the polymer molecules tend to grow too large to be useful, for instance, as a surfactant. Although molecular weight can be kept lower by the use of chain transfer agents, there remains the problem of including within the polymer both the polar and nonpolar components that are required in surface-active materials. This is especially true in view of the economic constraints that limit the chemistry and number of process steps required for making a suitable product.
Poly(vinyl acetate) and its hydrolyzed derivative, poly(vinyl alcohol), are two well known polymers that are available commercially in quantity, but it has been difficult to modify these materials to include acidic groups desirable for surfactant properties. The most common route attempted to date is through copolymerization of vinyl acetate with monomers containing acidic functionality. For example, in xe2x80x9cFunctional Modification of Poly(vinyl alcohol) by Copolymerization: 1. Modification with Carboxylic Monomers.xe2x80x9d Polymer, Vol. 38, No. 12, p.2933, (1997), Moritani and Kajitani suggest that multifunctional polymers useful as sizing in the paper and textile industries can be made by copolymerizing vinyl acetate and carboxyl-containing monomers and then hydrolyzing the acetate groups to alcohol as in the manufacture of poly(vinyl alcohol). The initiator used was AIBN. The copolymerization route to incorporate acidic groups in these polymers results in random distribution of the acidic groups along the polymer chain rather than in a terminal position in the molecule where acidic functionality would be more effective in enabling the polymer to serve as a surfactant.
Another way of incorporating acidic groups into a low molecular weight polymer molecule is described in U.S. Pat. No. 3,646,099, Dannals, (1972). This patent discloses making oligomers containing sodium sulfonate-terminated polymers by polymerizing a hydrophilic monomer, such as acrylic acid, by reductive polymerization using a relatively high proportion of sodium bisulfite as the reducing chemical. The monomer containing the hydrophilic group can be copolymerized with limited amounts of comonomer containing a hydrophobic group provided that the proportion of the hydrophobic comonomer does not exceed 60 mol percent and is preferably less than 30 mol percent of the polymerized monomeric units. The utility suggested for these polymers is as conductive agents. Although vinyl acetate is listed among a large group of suggested monomers containing hydrophobic groups, it is clear from the context of this disclosure that such monomers are to be used only as a minor comonomer, if at all. This is understandable since it has been found that vinyl acetate tends to react with bisulfite to form 1:1 adducts rather that polymerized products. For example, Mukherjee, et al. in xe2x80x9cBisulfite-Initiated Vinyl Polymerization in Aqueous Mediaxe2x80x9d, Makromolekulare Chemie, 80, p.208 (1964), investigated the use of bisulfites in the aqueous polymerization of certain vinyl monomers such as methyl methacrylate, ethyl methacrylate, methyl acrylate and styrene. These successful products were said to contain, on average, two sulfonate end groups per polymer chain. The reactions failed, however, in the case of vinyl acetate, acrylonitrile, methacrylate and acrylic acid. It was concluded that the reaction is monomer specific. Furthermore, standard industrial practice for preparing poly(vinyl alcohol) requires polymerization of vinyl acetate in methanol solvent, in which bisulfite is insoluble. Also, Schmitt, J. Org. Chem., 60, p.5474 (1995), described reaction of bisulfite with allyl groups but obtained only mixtures of sulfonate and sulfite groups in 1:1 adducts.
On the other hand, U.S. Pat. No. 4,360,632, Pinschmidt et al. (1982) discloses that vinyl acetate high polymers can be made in emulsion polymerization using a ketone bisulfite as a formaldehyde-free reducing agent in the initiator system. The products are high molecular weight polymers useful in latex form in the manufacture of non-woven goods. Two runs in which sodium bisulfite was used instead of ketone bisulfite are disclosed as control runs which gave poor results. It has remained, therefore, an unsolved problem of how one might incorporate sulfonate groups into oligomers of vinyl acetate and vinyl alcohol in such a way that these polymers are provided with enhanced surface active properties.
Two papers, P. Ghoshy, S. C. Chadha, A. R. Mukherjee, and S. R. Palit, J. Polym. Sci., Pt. A, 2, 4433-4440 (1964) and W. D. Hergeth, W. Lebek, R. Kakuschke, K. Schmutzler, Makromol. Chem. 192, 2265-2275 (1991), describe highly impractical syntheses of anionically terminated polymers or oligomers using persulfate and vinyl acetate by operating at very low monomer concentration (1% in the former, giving polymers of 475,000 Mn and, in the latter, a very dilute delay feed summing to 0.05 to 0.8% polymerized vinyl acetate on water at the end of the reaction to give 3000 Mn oligomer). In both papers, the authors describe the products as made by termination (instead of transfer as in our process) and, although the latter believe their chains contain one sulfate end group, the former authors measure 1.3 to 1.8 sulfate endgroups per chain. The sulfate endgroups are described by Ghoshy, et al. as hydrolytically unstable (p.4434) relative to sulfonate endgroups and would not survive hydrolysis to prepare a vinyl alcohol oligomer. The surfactant properties of the unhydrolyzed oligomer in the second article appear significantly different than what we have measured.
A. B. Moustafa, A. A. Abd El Hakim, G. A. Mohamed, J. Appl. Poly. Sci. 63, 239-246 (1997) reported emulsion polymerization of vinyl acetate at low solids (only 10% VAc on water) using a 1:1 molar ratio of persulfate and bisulfite without surfactant or cosolvent. They obtained poor conversions with 2.8 wt % persulfate on monomer, but could exceed 90% conversion with an exceedingly high 5.6 wt % persulfate on monomer. Their product was a stable, presumably high molecular weight emulsion polymer, rather than a dispersible oligomer.
According to our invention, a product is provided which is an oligomer of a vinyl ester of an organic acid having 2 to 18 carbon atoms, the molecules of the oligomer being terminated at one end by a sulfonate group. Our invention also provides a vinyl alcohol oligomer having a terminal sulfonate group. Such an oligomer is a derivative of a vinyl ester oligomer in which at least a portion of the ester moieties of the vinyl ester have been hydrolyzed to hydroxy groups. Our invention also provides a vinyl acetate or vinyl alcohol cooligomer with acid or quaternary amine containing monomers and having a terminal sulfonate group. These oligomers are unique in having one strong acid anion at one end of the molecule and adjustable levels of weak or strong acid anions or permanent cations along the chain. We have found that vinyl esters, especially vinyl acetate, can be polymerized in aqueous and partly aqueous media using a bisulfite chain transfer agent to form these useful oligomers containing terminal sulfonate groups, provided certain process procedures are followed.
In the process of our invention, a polymerization mixture is formed containing water, preferably with a cosolvent such as methanol, the vinyl ester, the bisulfite, preferably sodium or ammonium bisulfite, and a free radical initiator, preferably an oxidant which can serve as a component of a redox initiator. This mixture can be formed without the addition of an emulsifying agent as is normally the practice in preparing systems for emulsion polymerization. Although emulsifiers can be added, it is preferred that they not be used. As the product of the process is formed, the sulfonate-terminated oligomer, if sufficiently low in molecular weight, remains largely water soluble and acts as a compatibilizer in the system. If higher molecular weight poorly water soluble oligomer is desired, a cosolvent is used to aid in maintaining a one phase or effectively one phase mixture, in which both bisulfite and monomer are sufficiently soluble or miscible. In these ways it is possible to avoid the formation of partitioning micelles and phase separated hydrophobic particles which are typical of emulsion or dispersion polymerizations.
This mixture is subjected to polymerization conditions of temperature and agitation for a period of time during which vinyl ester is fed to the mixture at a rate such that a vinyl ester rich/bisulfite poor phase is not formed. In all aqueous or low cosolvent systems, this involves xe2x80x98starve feedingxe2x80x99 the vinyl ester at a rate such that the concentration of the vinyl ester in the mixture does not go over 3 weight percent. Also during this period the level of the bisulfite chain transfer agent in the mixture is controlled so that the average degree of polymerization (DP) of the product oligomer containing a single sulfonate end group per chain does not exceed a DP of 200 (number average molecular weight of 17,280 for vinyl acetate). This product is either recovered from the mixture or the mixture is converted to a condition suitable for hydrolysis of the oligomer to form the sulfonate-terminated vinyl alcohol oligomer product. For the production of the hydrolyzed oligomer, the bisulfite level should be such that the average degree of polymerization of the end product does not exceed 200.
According to our invention, vinyl esters of organic acids having 2 to 18 carbon atoms are polymerized in the presence of a bisulfite as a chain transfer agent in a virtual single-phase system. The bisulfite can also serve as the reductant along with an oxidant in a redox initiator used in the polymerization which is carried out in the presence of water, ideally with a cosolvent such as an alcohol, preferably methanol.
The polymerization is carried out in a virtual single phase which effectively maintains the proper ratios of vinyl ester to bisulfite in the reaction. This single phase system is maintained through the delay addition of monomer, by controlling the bisulfite level so that the oligomer or polymer formed is relatively low in molecular weight, and through the judicious use of appropriate solvents and solvent mixtures, preferably with no external surfactant added to the polymerization mixture. In this way micelles that tend to partition the polymerizing hydrophobic monomer from the water-soluble bisulfite are not formed, or at least are minimized in the reaction mixture. To further minimize partitioning of vinyl acetate and bisulfite into separate phases, the vinyl ester and comonomer, if any, are fed to the system in a continuous, or intermittent or semi-continuous manner which maintains a controlled proportion of bisulfite in relation to the vinyl ester. The feeding of the monomer is controlled so that during the polymerization the monomer does not significantly phase separate from a bisulfite containing phase into a bisulfite depleted organic phase. It is preferred to use vinyl esters of organic acids having 2 to 12 carbon atoms and particularly the vinyl esters of acetic, propionic and tert-butyric acids. As the polymerization progresses, the product sulfonate-terminated oligomer begins to function as a hydrophilic cosolvent in the system and can create an organic phase which is swollen with water, bisulfite, and any cosolvent that is present. Although this phenomenon may appear as converting the polymerization system from single phase to two phase, such is not actually the case in the sense normally understood for two phase, micelle partitioned emulsion (dispersion) polymerization. Any organic phase which forms due to the production of sulfonate-terminated oligomer serving as a compatibilizer does not exclude the bisulfite from the monomer because such organic phase is sufficiently swollen with water or water and cosolvent that the water-soluble bisulfite is available to the monomer in effective concentrations. Likewise, cosolvent and dissolved oligomer in any water rich phase prevent excessive depletion of vinyl ester monomer from that phase. This is unlike the situation that exists in a typical emulsion system where most of the vinyl ester is present in a salt depleted hydrophobic phase and is converted to high polymer.
The oligomers prepared by the process of the invention include a fairly broad range of molecular weights and can be characterized as low, medium and high molecular weight oligomers, all of which, however, find utility in various applications as surface-active agents. By xe2x80x9coligomersxe2x80x9d as used in defining this invention, therefore, we mean polymerized species which are not mere adducts of a monomer and a sulfonate radical, but are polymers having an average of from two to less than 200 monomer units (DP less than 200) and which can have, for vinyl acetate, number average molecular weights (Mn) as high as about 17,000. These oligomers preferably range from dimers and trimers to oligomers having a Mn below 15,000 for the oligomers of vinyl esters and below 7,500 for the vinyl alcohol oligomers (calculated on fully hydrolyzed PVOH). Even more preferably the Mn for the oligomers of vinyl esters lie in the range of 350 to 8000, and for the oligomers of vinyl alcohol in the range of 175 to 6000. Oligomer molecular weights determine solubility properties of the products, with unhydrolyzed oligomers having a Mn up to about 2000 showing water miscibility before hydrolysis and higher molecular weight oligomers showing water dispersibility and ready alcohol miscibility or solubility.
Molecular weight of the oligomer can be controlled by adjusting the bisulfite to monomer ratio and also by using chain transfer cosolvents, such as tetrahydrofuran or isopropanol. Control of molecular weight can be assisted through the adjustment of the levels of one or more of the vinyl ester, initiator, or cosolvent in the polymerization mixture, and also by changing the temperature. In general, increasing the monomer level will increase molecular weight while increasing initiator level, cosolvent level or temperature will decrease the molecular weight of the product.
The vinyl esters can be copolymerized with up to 30 mol percent of various comonomers of the type that are conventionally copolymerized with vinyl acetate. For example, suitable comonomers include other vinyl esters suitable for homopolymerization, especially esters of the C11 to C16 neoacids; sodium vinylsulfonate (SVS); diallyldimethylammonium chloride; maleic anhydride; acidic monomers such as acrylic, crotonic, acrylamidopropanesulfonic (AMPS), itaconic, methacrylic, maleic or fumaric acid, neutralized, for example with ammonia or an alkali metal hydroxide to a pH of 3 to 5; acrylamide and substituted acrylamides such as N-methylolacrylamide; vinyl chloride; ethylene; maleate or fumarate esters or neutralized half esters; amides or half-amides; amideesters; acrylic or methacrylic esters of C1 to C18 alcohols; functional (meth)acrylates such as hydroxyethyl and diethylaminoethyl acrylate; acrylonitrile; allyl esters; N-vinylamides such as N-vinylpyrrolidone, N-vinylformamide, and N-vinylacetamide; and the like. If comonomers are used, it is preferred that their proportion with relation to the principal monomer fall within the molar ratios of 1:20 to 1:9, comonomer to vinyl ester.
The bisulfite chain transfer agent can be any bisulfite compound which effects chain transfer in free radical initiated polymerization in aqueous media. Preferred bisulfites are ammonium or sodium bisulfites or metabisulfites since these compounds are readily available and easily handled. Also suitable, however, are other counterions such as alkaline earth salts and alkylammonium salts. Other sources of bisulfite can be used, such as sulfite salts converted to bisulfite by partial acidification; or sulfur dioxide dissolved in water and partially neutralized to bisulfite.
In preparing the oligomers of the invention, the amount of bisulfite chain transfer agent used in relation to the monomer polymerized depends upon the desired molecular weight of the oligomer. The molar ratio of vinyl ester to bisulfite should be at least 2 to 1 and can be as high as 50 or 60 to 1 or even higher. This ratio is a major variable in determining molecular weight of the oligomer, but pH of the system is also a consideration. The reaction proceeds best within a pH range of 3 to 7 where the bisulfite ions are not excessively converted by equilibrium to H2SO3 or sulfite. It is preferred to keep the pH of the system in the range of 4 to 6. Changes in the pH during the course of the reaction can also affect the sulfite/bisulfite ratio and are desirably controlled, for example by the use of a buffer. Sodium bicarbonate is quite effective for this purpose, but other systems known to be useful for buffering within the desired range can also be used. We have found that the pH values of 8.0 and above or below 3.0 are unfavorable for achieving low molecular weights for the oligomers, probably because the concentration of bisulfite becomes too low to sustain the required chain transfer function. In batch operations the bisulfite can be added either initially or continuously during the reaction.
Initiators used in the process of the invention can be any of the free radical polymerization initiators known in the art to be suitable for polymerizing radically polymerizable monomers. These include both thermally activated radical initiators and redox systems. Examples of suitable initiators include inorganic peroxidic materials, such as potassium, ammonium or sodium persulfate; hydrogen peroxide; azo compounds such as azo-bis-isobutyronitrile; organic peroxides such as peracids, for example peracetic acid, peranhydrides such as benzoyl peroxide, peresters such as t-butyl perneodecanoate, t-butyl hydroperoxide, and oxygen itself. Redox systems include hose based on a combination of an oxidizing agent, a reducing agent and a trace amount of an appropriate transition metal. Metal ions such as iron, copper, nickel, or chromium, and, most preferably, iron (II) or iron (III) salts plus an oxidizing agent such as hydrogen peroxide, t-butyl hydroperoxide, or K2S2O8 are also suitable choices. Since bisulfite is a reducing agent, addition of an additional reducing agent to form a redox system would be redundant but optional. Persulfate is the preferred initiator. The initiator, especially one operating as a part of redox system, is preferably added to the polymerization mixture continuously at a level effective to control monomer conversion rate. Suitable amounts are 0.01 to 5 weight percent of the monomer used, and preferably the initiator is added in an amount corresponding to 0.1 to 1.5 weight percent of the monomer.
The solvent system selected for the telomerization of the vinyl ester must be one in which both the vinyl ester and the bisulfite compound are at least somewhat soluble. Water or water/alcohol mixtures are preferred. Alcohols such as methanol, ethanol, 1-propanol or water miscible solvents or cosolvents such as acetonitrile or tetrahydrofuran are also suitable choices. The most desirable solvent is water alone or with methanol added. The system is operable in a predominantly alcohol media but there must be sufficient water present to dissolve the bisulfite. Preferably the volume ratio of water to methanol is in the range of 0.2/1 to 10/1. Generally the use of water alone or with up to 5 volume percent methanol is suitable for producing very low molecular weight oligomers, for example, oligomers with a number average molecular weight (Mn) of less then 1200. The use of higher proportions of methanol, in general, is needed for oligomers of higher molecular weight; within narrow ranges, increasing the methanol level can lower the molecular weight of the oligomer, other conditions being equal.
Although the starting reaction mixture is in a single phase, there is a tendency for more organic rich phase to develop due to limited miscibility of the product oligomer. This can be offset by using higher levels of alcohol or other organic cosolvent to maintain a single-phase mixture as the ratio of hydrophobe (e.g., vinyl acetate) to hydrophile (e.g. the bisulfite ion) is increased. Final oligomer to solvent ratios can vary from less than 10 weight percent to 50 percent and higher, with the higher ratios preferred for economic reasons and the lower ratios for obtaining very low molecular weights. Oligomer levels of 30 to 55 weight percent are preferred. Maintaining a single phase in the polymerization mixture has the further advantage of producing oligomers with narrower polydispersities. At the end of the reaction, however, conversion of the mixture into a two-phase system, for instance by cooling, simplifies separation of the oligomer from the solvent.
The temperature of the reaction can vary over a broad range but is generally in the range of about 20xc2x0 C. to 100xc2x0 C. Preferred temperatures for ease of operation are 25 to 75xc2x0 C. and most preferably about 60 to 70 0C. Temperatures outside these ranges are operable. The reaction time is usually about 1 to 10 hours for batch reactions and in continuous operations the reactor turnover times are in the range of 1 to 10 hours also.
The process of our invention is able to make sulfonate-terminated oligomers of vinyl esters rather than high molecular weight polymers in which the effect of any terminal sulfonate groups would be lost. While not to be bound by theory, we believe that the reason for this is that the operative ratios of bisulfite to the monomer in the rapidly polymerizing phase(s) are kept higher in the process of the invention than the ratios found in other polymerization processes. This is accomplished by starve feeding monomer to the system and controlling the phase condition of the polymerization mixture throughout the reaction. This condition is defined for the purpose of our invention as a xe2x80x9cvirtual single phasexe2x80x9d. This term includes both an actual single phase and a pseudo two-phase system in which any organic phase that appears to separate from the solvent phase is sufficiently swollen with water or water and cosolvent that the bisulfite is soluble in the organic phase. There is thereby little or no partitioning effect that would otherwise tend to separate the monomer and bisulfite. At the start of the reaction both the monomer and the sulfite ion are soluble in the solvent, be it water or a water/alcohol mixture. This mutual solubility continues in the solvent-swollen organic phase as the oligomer chains grow. Vinyl acetate in particular possesses an attractive combination of water solubility, polymer hydrophilicity, monomer reaction rate with bisulfite, and rate of self polymerization to facilitate the successful synthesis of the oligomers of the invention.
It is clear that the relative solubility of monomer and bisulfite in the process of the invention is quite unlike prior art vinyl acetate emulsion polymerization conditions in which there are higher vinyl acetate levels, no cosolvent and lower or no bisulfite proportions. In such systems short growing polymer chains partition into an organic phase either by adsorption into a preformed hydrophobic particle or by formation of a micelle. When this happens the bisulfite ions that are preferentially soluble in the aqueous phase tend to be excluded from the organic-phase where the polymer is continuing to grow in a monomer rich, bisulfite depleted condition. Such a condition produces high molecular weight polymer. Eventual chain transfer generates new non-sulfonate terminated chains that also grow to high molecular weights. The poor solubility of these hydrophobic chains in water essentially suppresses their back migration into the aqueous phase where they could be terminated by bisulfite to reinitiate a telomerization reaction. The hydrophobic chains likewise swell or dissolve in monomer in the hydrophobic phase, reducing monomer concentration in the bisulfite rich aqueous phase and suppressing bisulfite concentrations in the monomer phase.
The sulfonate-terminated vinyl alcohol oligomer products of the invention are made by hydrolysis of the sulfonate-terminated poly(vinyl esters). The hydrolysis can be partial if desired, or hydrolysis can be essentially complete under suitable conditions. If the vinyl alcohol oligomer product is the objective from the beginning, it is logical to work with the oligomers of vinyl acetate. No advantage is to be gained by using the vinyl esters of higher acids because the acidic moiety of the ester is removed on hydrolysis of the polymer. Vinyl acetate is the most readily available vinyl ester in addition to being the easiest of the monomers to work with. Comonomers can be used, however, since these introduce functionality into the oligomer structure that is not destroyed on hydrolysis. Hydrolysis of poly(vinyl acetate) to form poly(vinyl alcohol) is a process well known in the art and conventional conditions can be employed in this aspect of our invention. The process is accomplished in an easily controlled process by mixing oligomer with methanol, optionally filtering to remove a small amount of solids, and adding acid or base as a hydrolysis catalyst. This provides a readily water-soluble, sulfonate-terminated vinyl alcohol oligomer of the desired molecular weight and hydrolysis level, depending upon the amount of catalyst used, the hydrolysis time, and the temperature. Usually the product is a solid. The products have good color with very rapid cold water solubility and give extremely low viscosity solutions.
Hydrolysis of the oligomer from the vinyl acetate telomerization is best accomplished in a high alcohol solution to increase rate and minimize coproduction of acetate ion (or acetic acid under acid conditions). This can be accomplished, for instance, by minimizing water in the initial reaction or by removal of most of the water of the polymerization mixture under reduced pressure. An excess of methanol, or optionally ethanol, is added to give a ratio of 5 to 80 weight percent and preferably 30 to 50 weight percent of the oligomer. Following this step an effective level of acid or base catalyst is added. Such catalysts are well known in the art of poly(vinyl alcohol) synthesis. Sodium or potassium hydroxide or methoxide are examples of base catalysts and hydrochloric, nitric, sulfuric and methanesulfonic acids are examples of acid catalysts. The effective amount of catalyst can vary from less than 0.1 mol percent to over 10 mol percent based on the oligomer. This amount depends upon the water left in the system, which can be over 25 percent, but is preferably less than 15, and most preferably less than 6 percent of the mixture, as well as on the time and temperature of the reaction. Usually the time is from one to 30 minutes in water free systems and 15 minutes to 8 hours in water containing alcohol media. The temperature can vary from less than 25 to over 100xc2x0 C. Preferably the temperature is in the range of 35 to 70xc2x0 C. These conditions will also depend upon the molecular weight of the oligomer and the desired level of hydrolysis. When an alcohol solvent is used, the product precipitates or phase separates as a white or off-white solid which can be isolated from the alcohol, coproduced acetate ester, and other salts. Hydrolysis levels can vary widely as desired for particular applications, depending upon the conditions and the size and type of oligomer. Hydrolysis levels can range from as low as about 10 percent to 100 percent. Other possible conversions include hydrolysis in water or high water/solvent mixtures by the addition of acid or base These methods can be attractive when hydrolysis byproducts do not detract from product performance or when lower levels of hydrolysis are desired.